It has been recently considered to incorporate a particulate filter in an exhaust pipe for collection of particulates in an exhaust gas, a selective reduction catalyst capable of selectively reacting NO with ammonia even in the presence of oxygen being arranged downstream of the particulate filter, a urea water as a reducing agent being added between the selective reduction catalyst and the particulate filter, thereby achieving simultaneous reduction of the particulates and NOx.
In such a case, the addition of the urea water to the selective reduction catalyst takes place between the particulate filter and the selective reduction catalyst; thus, in order to secure a sufficient reaction time for thermal decomposition of the added urea water into ammonia and carbon dioxide gas, it is necessary to lengthen a distance from an added position of the urea water to the selective reduction catalyst. There is, however, a problem that the spaced-apart arrangement of the particulate filter and the selective reduction catalyst with a sufficient distance in between would significantly impair a mountability on a vehicle.
Accordingly, devised was an exhaust emission control device as shown in FIG. 1 comprising a particulate filter 3 for capture of particulates in exhaust gas 1 from an engine and a selective reduction catalyst 4 downstream of the particulate filter 3 and having a property capable of selectively reacting NOx with ammonia even in the presence of oxygen; the particulate filter 3 and the selective reduction catalyst 4 are encased by casings 5 and 6, respectively, and incorporated in parallel with each other in an exhaust pipe 2 through which the exhaust gas 1 from the engine flows; and an exit end of the particulate filter 3 is connected with an entry end of the selective reduction catalyst 4 through an S-shaped communication passage 7 so that the exhaust gas 1 discharged from the exit end of the particulate filter 3 is turned back in an opposite direction and is guided to the entry end of the adjacent selective reduction catalyst 4.
The communication passage 7 is the S-shaped structure comprising a gas gathering chamber 7A encompassing the exit end of the particulate filter 3 to gather the exhaust gas 1 just discharged from the exit end through substantially perpendicular turnabout, a mixing pipe 7B for extraction of the gathered exhaust gas 1 in the chamber 7A in a direction opposite to that of the exhaust gas flow in the particulate filter 3 and a gas dispersing chamber 7C for dispersion of the exhaust gas 1 from the mixing pipe 7B through substantially perpendicular turnabout and for encompassing of the entry end of the selective reduction catalyst 4 to guide the dispersed exhaust gas 1 to the entry end of the catalyst. The entry end of the mixing pipe 7B is centrally provided with a urea water adding injector 8 (urea water adding means) for addition of the urea water into the mixing pipe 7B and directed to the exit end of the mixing pipe 7B.
In the example illustrated, an oxidation catalyst 9 is installed inside the casing 5 before the particulate filter 3 to perform an oxidation treatment of unburned fuel in the exhaust gas 1. An ammonia reducing catalyst 10 is installed inside the casing 6 after the selective reduction catalyst 4 to perform an oxidation treatment of surplus ammonia.
With such a configuration employed, the particulates in the exhaust gas 1 are captured by the particulate filter 3; and, downstream thereof, the urea water is added in the exhaust gas 1 midway of the mixing pipe 7B by the urea water adding injector 8 and is thermally decomposed into ammonia and carbon dioxide; then, on the selective reduction catalyst 4, NOx in the exhaust gas 1 is well reduced and purified by the ammonia. As a result, the simultaneous reduction of the particulates and NOx in the exhaust gas 1 can be achieved.
In this case, the fact that the exhaust gas 1 discharged from the exit end of the particulate filter 3 is turned around in the opposite direction by the communication passage 7 and then guided to the entry end of the adjacent selective reduction catalyst 4 ensures a long distance from the added position of the urea water to the selective reduction catalyst 4 and ensures a sufficient reaction time for production of ammonia from the urea water.
Moreover, the fact that the particulate filter 3 and the selective reduction catalyst 4 are arranged in parallel with each other and the communication passage 7 is arranged between and alongside of the particulate filter 3 and selective reduction catalyst 4 makes an overall configuration compact in size, thereby substantially improving the mountability on the vehicle.
A conventional technology document related to this type of exhaust emission control device with a particulate filter 3 and a selective reduction catalyst 4 arranged in parallel with each other and interconnected through an S-shaped communication passage 7 is, for example, the following Patent literature 1.